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The reasons for the explosion of 18650 lithium-ion batteries

1. Internal short circuit: Structural or process defects leading to insertion of the pole ear, insufficient wrapping of the diaphragm, burrs, diaphragm folding, etc. Insertion of pole ears often occurs in structures with thick batteries and internal parallel connections. Insufficient insulation at the pole ear position can easily lead to acute internal short circuits and combustion or explosion during subsequent product installation or use. However, this situation is almost rare now, unless external forces are involved.

2. External short circuit: Explosion after combustion caused by external short circuit of lithium-ion batteries below 1AH is not common, usually accompanied by bulging or simply burning off the pole ears. Personal analysis has shown that combustion is caused by external short circuit of power lithium-ion batteries or high-capacity mobile phone batteries. This involves improper use by customers when installing metal outer boxes without considering pole ear insulation, resulting in abnormal battery smoke and combustion at the installation site; Insufficient structural fixing measures are also common in power lithium batteries (due to inadequate consideration). During user use, the shaking of the battery pack can cause damage to the external insulation protection film of the battery or damage to the connecting wire skin, ultimately leading to short circuit combustion.

3. Overcharging explosion: This is the most dangerous and feared by the company, but it still occasionally occurs. Based on our understanding, there are two points: a. Users do not use matching chargers as required, which damages the protective circuit. Additionally, users often charge for an unlimited period of time, which is difficult to avoid; b. The 18650 lithium-ion battery configuration is unreasonable and the protective plate is ineffective, which can also cause a complete explosion. The same applies to a single battery.

Overcharged

Overcharging of 18650 lithium batteries is a common cause of explosion. Lithium ion batteries must operate within the specified voltage range, otherwise there may be a safety hazard of explosion and fire. For example, ternary lithium batteries typically have a voltage range of 2.5V to 4.2V; The conventional voltage for lithium iron phosphate batteries is 2.0V~3.65V.

When charging a 18650 lithium battery, the lithium ions from the positive electrode move to the negative electrode and are embedded in the network structure of the negative electrode material. If the lithium-ion battery is overcharged, it is easy to cause lithium precipitation at the negative electrode, causing lithium crystallization and piercing the separator, which can cause an internal short circuit in the battery cell and trigger an explosion.

That's why lithium-ion batteries must be equipped with overcharge protection circuits during charging to provide overcharge protection. The lithium battery protection board monitors each voltage string of the lithium-ion battery pack. If it exceeds the overcharge voltage, the lithium battery protection board will cut off charging, playing a role in protecting the lithium-ion battery.

If an overcharge explosion occurs in a lithium-ion battery pack, most of the reasons for this are due to individual instances of 0V voltage in the lithium-ion battery string, or abnormal protection boards that cannot provide overcharge protection for the lithium-ion battery; Another issue is the incorrect use of the charger.

For example, a 36V lithium-ion battery pack requires 10 strings of ternary lithium. If the voltage of 2 strings of battery cells becomes 0V, the charging voltage of the charger will directly charge the remaining 8 strings with 42V. The protection board function is abnormal and cannot provide overcharge protection function. The average voltage of 8 series of battery cells will be charged by the charger to 5.25V or above (42V/8 series). This can lead to overcharging of lithium-ion batteries, causing accidents such as explosions.

If the charger is used incorrectly and the lithium-ion battery pack is not charged according to the prescribed dedicated charger, it can also cause abnormalities in the lithium-ion battery protection board and overcharge the lithium-ion battery pack.

For example, for a 36V lithium-ion battery pack, the output voltage of a dedicated charger should be 42V. If a 72V lithium-ion battery pack charger is used (with a charging voltage of 84V), and the charging voltage of 84V is directly loaded onto the 36V lithium-ion battery pack, the components of the protection board are easily damaged and the function is prone to failure. At this point, the lithium-ion battery pack will be charged without protection, and the voltage of each series of battery cells will be charged to above 8.4V (84V/10 series). The full charging voltage of ternary lithium is 4.2V, and 8.4V voltage is relatively dangerous for ternary lithium single cell batteries, which can easily cause explosions.

So, please use lithium-ion batteries correctly, use lithium-ion battery protection boards correctly, and use lithium-ion battery chargers correctly.

Short circuited

Short circuits in 18650 lithium-ion batteries, including internal and external short circuits, can cause explosions in 18650 batteries.

There are several reasons for internal short circuits, one of which is due to production process issues where debris remains inside. During the use of lithium-ion batteries, there is an internal short circuit between the positive and negative electrodes, leading to an explosion. Another reason is that external punctures or squeezing cause internal material short circuits, leading to explosions.

The external short circuit is caused by direct contact between the positive and negative electrodes of lithium-ion batteries. For example, if the load equipment malfunctions, the circuit is short circuited, and the lithium-ion battery protection board fails to cut off the circuit in time, continuing the short circuit will cause an increase in internal air pressure of the lithium-ion battery, leading to an explosion. The reasons for external short circuits include short circuits caused by vibration, short circuits caused by compression and collision, and so on.

The explosion and fire caused by a short circuit in the 18650 lithium-ion battery pack were more powerful, with sparks occurring simultaneously with the explosion.

So, please protect the 18650 lithium-ion battery, from assembly process, packaging and shipping, transportation, etc., paying attention to each process to prevent the lithium-ion battery from being short circuited.

high temperature

Excessive temperature can also cause explosions in 18650 lithium-ion batteries. The normal operating temperature range for lithium-ion batteries is 0-45 ℃ for charging; Discharge -20~65 ℃; Store at 0-45 ℃.

The lithium-ion battery protection board is generally set to charge at a high temperature of 50 ℃ for protection; Discharge high temperature protection at 70 ℃. If there is no high-temperature protection, continuing to discharge or store lithium-ion batteries can cause explosions in lithium-ion batteries.

The internal materials of lithium batteries will deteriorate above 120 ℃, such as the separator of the internal materials of the battery cell. The material used for the separator of conventional battery cells is relatively ordinary, and the separator will shrink above 120 ℃. Once the separator shrinks, the positive and negative electrode materials will short-circuit and directly contact, quickly causing a high-temperature internal short circuit and instantly triggering an explosion of lithium-ion batteries.

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